The packaging container containing liquid food, such as milk and soft drinks, has been manufactured by using a web-like packaging material or a packaging container blank-like packaging material and sealing the predetermined part of each packaging material with heat sealing or ultrasonic sealing, etc. For instance, the packaging container was formed by making the packaging material into a tube-like material, sealing the material in the vertical direction using the vertical sealing device as the 1st sealing device, sealing the material transversally using the transversal sealing device as the 2nd sealing device at the predetermined length while filling liquid food into the material, cutting the material to form the original packaging container with the brick-like shape, and then forming the original packaging container again to complete forming the packaging container.
The packaging material is sealed by pinching with the predetermined pinching pressure and applying heat on the surface to melt the resin. However, poor sealing may occur when the melted resin escapes from the sealed part and sufficient amount of resin does not remain on the sealed part according to the pinch pressure, sealing temperature, and resin type. With poor sealing, liquid food in the packaging container may leak or the quality of liquid food may deteriorate since air enters into the packaging container.
Thus, an operator must select a sealed packaging container and discharge liquid food from the packaging container, unpack the packaging container, and inspect the sealed part of the packaging container by viewing from the inside of the packaging container.
However, with the above-mentioned sealed condition inspection, the sealed condition cannot be judged properly since the judgements vary according to the operator's subjectivity.